The present invention relates in general to photolithography techniques for producing semiconductor devices and the like, and in particular to a new and useful method of forming a pattern in an insulator layer for a semiconductor structure utilizing a photosensitive polyimide which becomes soluble when exposed to light.
In the fabrication of multi-level metal-insulator integrated circuit structures, a polyimide known as Kapton (a trademark of DuPont) has proven to be a good insulator when applied between metallization layers in integrated circuit structures. This is because of the high thermal stability, chemical resistance and dielectric properties of Kapton polyimide. Kapton polyimide has the following formula: ##STR2##
An intermediate polyamic acid which is subjected to heat to form the Kapton polyimide, is itself soluble and can be spun into films which can be cured into the insoluble polyimide structures. Once the material is cured, it is generally insoluble and infusible and is extremely thermally stable. The insolubility and the infusibility of the Kapton polyimide requires that patterning of the polyimide layers be accomplished indirectly by photoresist technology. This process entails the spinning and curing of the polyimide layer, formation of a polysulfone lift-off layer and deposition of an SiO.sub.2 masking layer followed by a top layer of resist coating.
The pattern is defined by either electron beam or optical lithography and the underlying layers are etched with reactive ion etching. Metal is deposited onto the pattern and the polysulfone and excess are lifted off with a solvent.
If the polyimide layer itself could be made intrinsically photosensitive, the formation of a pattern in the polyimide layer would be greatly simplified. Several photonegative polyimide systems have been developed which utilize photosensitive polyimides. These are generally made by the reaction of the corresponding polyamic acid with a photosensitive group. In the most common case, the intermediate polyamic acid is partially esterified with photo-crosslinkable alcohols. Irradiation of these esters causes them to become insoluble and enables them to be used to form negative images upon treatment with solvent. After imaging, the films are thermally converted to the polyimide which itself is not sensitive to light.
A photopositive system would be more desirable because of the swelling attendant upon solvent development of negative images. A photopositive polyimide containing photosensitive sulfonium salt units has been described in a article by Crivello et al. entitled "Synthesis and Characterization of Photosensitive Polyimides", Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 25, 3293-3309 (1987).
A polyimide resin which has good transparency and is useful to produce molded products with substantially no coloring and good thermal resistance, is disclosed in European Patent Application 0 130 481 to Noriaki et al. This reference does not consider or discuss the possibility of photosensitivity for the polyimide product.